1. Field of the Invention
The present invention relates to dither processing.
2. Description of the Related Art
In an electrophotographic printer, a method for forming a latent image on a photosensitive drum by using an exposure head is generally known. The exposure head with a light emitting diode (LED) or an organic electroluminescent (EL) irradiates the photosensitive drum with light to form the latent image. The exposure head includes a light emitting element group arranged in a longitudinal direction of the photosensitive drum, and a rod lens array for forming an image on the photosensitive drum with the light of the light emitting element group. Herein, a length of the light emitting element group is determined according to a width of an image region on the photosensitive drum, and a distance between pixels is determined according to resolution of the printer. The photosensitive drum corresponding to A3 size as an image region width has a length of approximately 310 mm, and the light emitting element group includes approximately 15,000 light emitting elements when a resolution is 1,200 dots per inch (dpi) (the approximate number of 15,000 is calculated by (310×1,200÷25.4)). In this case, a distance between the light emitting elements, that is, a distance between pixels is 21.2 μm (digits after the second decimal place are omitted).
A printer employing such an exposure head uses fewer components than a laser scanning-type printer which performs polarization scanning of laser beams by a polygonal mirror drive motor. Accordingly, with the fewer number of components, size and costs of the printer employing the exposure head are readily reduced. However, the light emitting element such as an LED and an organic EL has a characteristic in which an amount of light decreases over time. Especially, a light amount of the organic EL decreases faster. In a case where frequency of use of each light emitting element differs, deterioration progress of each element varies, causing variation in the light amount. Consequently, a poor-quality image such as an image having streaks is generated.
Japanese Patent Application Laid-Open No. 2008-87196 discusses an arrangement of a light emitting element group and an addition of a dummy pixel to the top of image data to deal with such a problem. The light emitting element group is arranged to have a length that exceeds a width of an image region as a printing target. The dummy pixel is added to the top of image data when a different page is printed. Such an addition of the dummy pixel to the top of image data changes allocation of effective light emitting elements. This prevents lighting unevenness in a pixel position in a main scanning direction even if image data of the printing target is the same.
However, according to the method discussed in Japanese Patent Application Laid-Open No. 2008-87196, for example, allocation of the light emitting elements to be used is changed for each page. Although such a change in the allocation reduces a load of the light emitting element during continuous lighting, positional deviation occurs every page due to the addition of the dummy pixel.
This type of issue may occur not only in the light emitting elements such as an LED and an organic EL, but also in a printing apparatus including one or a plurality of elements per pixel like an ink discharge element for inkjet printing. However, the printing apparatus is not the only apparatus which is to solve this issue. In the present specification, an element such as a light emitting element and an ink discharge element used for printing is called a printing element.
For example, it may be preferable for a host computer, in which an application (e.g., PHOTOSHOP®) capable of performing dither processing is installed, to take countermeasures to cope with such an issue although the host computer does not necessarily execute a printing operation, because a dither processing result acquired by such application is often transmitted to the printing apparatus including such printing element.